1. Field of the Invention
This invention relates to an optical fibre management system, and in particular to an optical fibre splitter array sub-assembly for incorporation in the node of an optical fibre telecommunications network.
2. Related Art
In the United Kingdom, the telecommunications network includes a trunk network which is substantially completely constituted by optical fibre, and a local access network which is substantially completely constituted by copper pairs. Flexibility in the copper access network is provided at two points en route to the customer; firstly, at streetside cabinets serving up to 600 lines; and secondly, at distribution points serving around 10-15 lines. In total, the network has about 250,000 km of underground ducts, 83,000 cabinets, 3.1 million distribution points and 3.7 million manholes and joint boxes. Eventually, it is expected that the entire network, including the access network, will be constituted by fibre.
The ultimate Goal is a fixed, resilient, transparent telecommunications infrastructure for the optical access network, with capacity for all foreseeable service requirements. One way of achieving this would be to create a fully-managed fibre network in the form of a thin, widespread overlay for the whole access topography as this would exploit the existing valuable access network infrastructure. Such a network could be equipped as needs arise, and thereby could result in capital expenditure savings, since the major part of the investment will be the provision of terminal equipment on a `just in time` basis. It should also enable the rapid provision of extra lines to new or existing customers, and flexible provision or reconfiguration Of telephony services.
In order to be completely future proof, the network should be single mode optical fibre, with no bandwidth limiting active electronics within the infrastructure. Consequently, only passive optical networks (PONs) which can offer this total transparency and complete freedom for upgrade, should be considered.
The most common passive optical network is the simplex single star, with point-to-point fibre for each transmit and receive path, from the exchange head end (HE) to the customer network terminating equipment (NTE). This network design has been used throughout the world and meets all the access criteria. It involves high fibre count cables, and unique electro-optic provision at HE and NTE for each customer. The resulting inherent cost can only be justified for large business users, who generally also require the security of diverse routing, which increases the cost still further.
The advent of optical splitters and wavelength-flattened devices has enabled the concept of the PON to be taken one step further. These passive components allow the power transmitted from a single transmitter to be distributed amongst several customers, thereby reducing and sharing the capital investment. In 1987, BT demonstrated splitter technology in a system for telephony on a passive optical network (TPON), with a 128 way split and using time division multiplex (TDM) running at 20 Mb/s. This combination enabled basic rate integrated service digital network (ISDN) to be provided to all customers. In practice, the competitive cost constraint of the existing copper network precludes domestic customers from having just telephony over fibre, due to the high capital cost of equipment. This may change in the future. In the meantime, telephony for small business users (for example those having more than 5 lines) can probably break this barrier.
The wider range of services and higher capacity required by business customers makes a 32-way split more attractive for a 20 Mb/s system and this has been demonstrated by BT's local loop optical field trial (LLOFT) at Bishop's Stortford.
In summary, the use of splitter based PON architecture will reduce the cost of fibre deployment in the access network. When compared with point-to-point fibre, savings will result from:
(i) reducing the number of fibres at the exchange and in the network; PA1 (ii) reducing the amount of terminal equipment at the exchange; PA1 (iii) sharing the cost of equipment amongst a number of customers; PA1 (iv) providing a thin, widespread, low cost, fibre infrastructure; and PA1 (v) providing a high degree of flexibility, and allowing `just in-time` equipment and service provision.
Additionally, PON architecture can be tailored to suit the existing infrastructure resources (duct and other civil works).
Total network transparency will retain the option for future services to be provided on different wavelengths to the telecommunications, which for TPON is in the 300 nm window. By transmitting at other wavelengths, other services, such as broadband access for cable television and high definition television, or business services, such as high bit rate data, video telephony or video conferencing, can be provided. The huge bandwidth potential of fibre promises virtually unlimited capacity for the transparent network. Eventually, it will be possible to transmit hundreds of wavelengths simultaneously, as the development of technology in optical components, such as narrow band lasers, wavelength division multiplexers (WDMs), optical filters, fibre amplifiers and tunable devices, moves forward.
For this potential to remain available, and for the access network to be used to provide many and varied services, then it must be designed and engineered to provide very high levels of security and resilience. Even for simple POTS, advance warning and live maintenance are essential to limit disruption.
Resilience implies separacy of routing, and exploiting the existing infrastructure of underground ducts and other civil works is a prime requirement of the design philosophy. Analysis of this resource has indicated that separacy, from creating primary ring topographies, could be achieved by linking the spine cables which currently feed many primary connection points (PCPs) in the existing star type network.
In order to create rings from the existing star configurations, some localities will have existing ducts that will allow the link cables to be installed. In BT's suburban networks, analysis has shown that on average 60% of PCPs can be served on rings using existing ducts; and, by adding new ducts Links of 200 m or less, a further 30% can be covered. In some cases, there will be natural or man made boundaries where physical rings cannot be provided, and in these cases duplicate fibres in the same duct route, i.e. across rivers or over railway bridges, may be the only choice.
The architecture adopted for the PON topography will be influenced by transmission techniques, and the availability of suitable splitter components. Transmission options are simplex (two fibre paths), duplex, half duplex or diplex (single fibre path).
Simplex working increases the complexity of the infrastructure due to the two fibres per circuit required. However, it benefits from the lowest optical insertion loss, due to the absence of duplexing couplers; and the lowest return loss, since such systems are insensitive to reflections of less than 25 dBm with separate transmit and receive paths. Duplex and half duplex working each have an insertion loss penalty of 7 dB from the duplexing couplers, and diplex working replaces these with WDMs, with a reduced penalty of 2 dB.
In view of the long term aim to provide a total fibre infrastructure, and the present early state of passive technology components, it is considered beneficial to opt for simplex working and a relatively low level of split (.ltoreq.32) for PON networks.
In an optical fibre communications system, transient changes in optical attenuation can cause transmission errors. These changes are caused by transient bend loss at various points along the fibres of the system, and the extent to which traffic along a given fibre is disturbed is dependent on such physical variables as the total loss incurred and the duration of the transient. Transient losses occur mainly because of fibre handling and maintenance procedures, particularly in the regions of fibre splices. Thus, when multi-fibre splice trays are opened, and/or fibre is handled, attenuations of up to 10 dB can be observed. For example, a typical splice tray used in optical communications systems contains 24 splices, and handling any one of the splices for maintenance purposes causes transient losses in adjacent fibres. This problem is illustrated in FIG. 15 of the accompanying drawings, which plots the probability of the occurrence of error against the system margin, at both 1550 nm and 1300 nm for a procedure which includes opening a typical 24 fibre splice tray, and running a finger along the splices. Error loss measurements are made of the fibres at splice position 14, as this splice position is almost in the centre, and so is more susceptible to transient losses than other splice positions. As shown in FIG. 15, at the optimum operating position of the receiver making the error measurement (that is to say at a system margin of 0 dB) at both 1550 nm and 1300 nm there is a large percentage error occurrence as a result of the transient losses caused by the fibre handling. As the system margin is increased, the percentage error occurrence falls at both 1550 nm and 1300 nm, but there is still a significant percentage error occurrence at 1550 nm even as the system margin approaches the dynamic range (typically 15 dB) of the receiver. The normal operating position of the receiver is the optical power nominally detected at the receiver to achieve a bit error rate (BER) of 10.sup.-9 or better. The results at 1550 nm are far worse than those at 300 nm, due to increased bend sensitivity at 1550 nm and hence larger transients. This is potentially worrying if splice trays are to be installed in a system operating at 1300 nm with later provision for operation at 1550 nm. This is because there may be a point at which a system may operate with no handling errors at 1300 nm, but will show a serious handling error performance at 1550 nm due to the increased bend loss sensitivity of fibres at 1550 nm. This would lead to a need for an increased system margin at 1550 nm to compensate for the greater losses at 1550 nm. This is undesirable because it results in lower power budgets due to a higher required incident optical power.